1. Field of the Invention
The invention relates to communication systems. In particular, the invention relates to overlaying code division multiple access communication systems.
2. Description of the Related Art
The wireless medium is becoming one of the predominate means by which voice information and digital data are transferred in modern society. In wireless communication systems, it is common that a single base station transmits signals to a plurality of remote units. In order to combat the harsh multipath wireless transmission channel while providing signaling to a plurality of remote units, modulation and coding schemes which provide efficient data transfer, as well as user channelization, have been developed. In general, these schemes operate most efficiently when each signal from a base station is transmitted with synchronized timing. For example, in a typical code division multiple access (CDMA) system, the signals are distinguished from one another through the use of different codes. In the case of transmissions from a base station to a remote unit, these codes are typically an orthogonal set of codes such as, for example, Walsh functions. If the transmissions corresponding to one orthogonal channel become misaligned with the transmissions corresponding to another channel, the orthogonal nature of the codes is degraded and the transmissions may significantly interfere with one another.
FIG. 1 is an exemplifying embodiment of a terrestrial wireless communication system 10. FIG. 1 shows three remote units 12A, 12B and 12C and two base stations 14. In reality, typical wireless communication systems may have many more remote units and base stations. In FIG. 1, the remote unit 12A is shown as a mobile telephone unit installed in a car. FIG. 1 also shows the portable computer remote unit 12B and the fixed location remote unit 12C such as might be found in a wireless local loop or meter reading system. In the most general embodiment, the remote units may be any type of communication unit. For example, the remote units can be hand-held portable units, portable data units such as a personal data assistant, or fixed location data units such as meter reading equipment. FIG. 1 shows a forward link signal 18 from the base stations 14 to the remote units 12 and a reverse link signal 20 from the remote units 12 to the base stations 14.
In the discussion that follows, to aid in illustration, the invention is described with reference to a commonly known, wireless link industry standard. In fact, the generic principles of the invention can be directly applied to many multiple access communication systems. The discussion that follows assumes operation in accordance with the system described in TIA/EIA/IS-95-A and its progeny, the contents of which are incorporated herein by reference, published by the Telephone Industry Association entitled “Mobile Station-Base Station Compatibility Standard for Dual-Mode Wideband Spread Spectrum Cellular System” commonly referred to as IS-95.
In a typical wireless communication system, such as that illustrated in FIG. 1, some base stations have multiple sectors. A multi-sectored base station comprises multiple independent transmit and receive antennas as well as independent processing circuitry. The principles discussed herein apply equally to each sector of a multi-sectored base station and to a single sectored independent base station. For the remainder of this description, therefore, the term “base station” can be assumed to refer to either a sector of a multi-sectored base station or a single sectored base station.
In systems using IS-95, remote units use a common frequency bandwidth for communication with all base stations in the system. Use of a common frequency bandwidth adds flexibility and provides many advantages to the system. For example, use of a common frequency bandwidth enables a remote unit to simultaneously receive communication signals from more than one base station, as well as transmit a single signal for reception by more than one base station. The remote unit discriminates between the simultaneously received signals from the various base stations through the use of the spread spectrum CDMA waveform properties.
In a wireless system, maximizing the capacity of the system in terms of the number of simultaneous calls that can be handled is extremely important. If a minimum acceptable signal quality is specified, an upper bound on the number of simultaneous users which can communicate through a base station can be calculated. With some simplifications, the amount of power that a remote unit must transmit is given by Equation 1:                     P        =                                                            RN                0                            ⁡                              (                                                      E                    b                                                        N                    t                                                  )                                      req                                1            -                                          R                W                            ⁢                              (                                  N                  -                  1                                )                            ⁢                                                (                                                            E                      b                                                              N                      t                                                        )                                req                                      -                          α              ⁢                                                           ⁢                              R                W                            ⁢                                                (                                                            E                      b                                                              N                      t                                                        )                                req                                                                        Eq        .                                   ⁢        1            wherein:                N is the number of remote units operating within a common coverage area;        R is the average data rate, which is assumed the same for all remote units;        W is the spreading rate;        N0 is the thermal noise floor of the base station plus interference from non-power controlled sources;       (                  E        b                    N        t              )    reg        is the required energy per noise spectral density for a remote unit,        which is assumed equal for each remote unit; and        α is the interference coupling coefficient from all other coverage areas.The upper bound on the number of simultaneous users is commonly referred to as the pole capacity of a system and is given when the denominator of Equation 1 is equal to zero. The ratio of the actual number of users to the pole capacity is defined as the loading of the system. As the number of actual users approaches the pole capacity, loading approaches unity. A loading close to unity implies potentially unstable behavior of the system. Unstable behavior can lead to degraded performance in terms of voice quality, high error rates, failed handoffs and dropped calls. In addition, as loading increases, the required output power of the remote unit increases. Because the output power of the remote unit is limited, the size of the coverage area of the base station shrinks such that users on the outer edge of the no-load coverage area are no longer be able to transmit sufficient power to communicate with the base station at an acceptable signal quality when the base station becomes heavily loaded. Equation is specifically for the reverse link. However, there is a comparable equation for the forward link with comparable effects. For the forward link, as the loading increases, the required output power of the base station increases.        
For these reasons, it is advantageous to limit the number of users which access the system such that loading does not exceed a specified percentage of the pole capacity. One way to limit the loading of the system is to deny access to the system once the loading of the system has reached a predetermined level. For example, if the loading increases above 70% of the pole capacity, it is advantageous to deny requests for additional connection originations and to refrain from accepting hand-off of existing connections.
When two CDMA systems are operated within a common bandwidth, these same issues of loading as well as time and phase synchronization are still important. If the two systems are overlaid without paying attention to their underlying properties, resulting capacity can be quite low. This is particularly true for the forward link where the signals radiated by the base station are typically orthogonal. For example, the forward link wave forms described in the above-referenced IS-95 are orthogonal to one another. If the overlaid signal set is not orthogonal to the existing signal set, then the capacity reduction can be quite large. In order to maintain orthogonality, the two systems must be time synchronized with one another. In addition, the capacity of the two systems is also related in that the loading on the first system acts as interference to the second, thereby decreasing the capacity of the second system, and the loading on the second system acts as interference to the first, thereby decreasing the capacity of the first system.
Use of a pilot signal improves the capacity on the forward link. The pilot signal is used for acquisition and channel estimation such as, for example, timing, phase, power control, and received symbol weighting. The other signals transmitted by the base station have a known and constant phase alignment with respect to the pilot signal. To avoid transmitting multiple pilot signals, it is preferable to maintain the constant phase alignment with respect to the overlaid channels.
When deploying a second CDMA system in the presence of an existing system, the existing system is not typically configured with the capability of interfacing with a second type of system. Therefore, the existing system does not typically provide the outputs or accept the inputs which are required to achieve synchronization and co-ordinate loading between the two systems. In contrast, the new system can be designed with such an overlay configuration in mind. In order to reduce the cost of deploying a second overlay system, it is important to avoid modification of the existing system.
Therefore, there is a need in the art for a system and method for overlaying two CDMA systems on the same frequency bandwidth.